CANDELS galaxy colors

The red circles from left to right represent the measured average color of galaxies at a redshift of 4, 5, 6 and 7, where the bottom axis shows the time since the Big Bang. The light blue bar running through the center of the diagram shows the color of the local galaxy NGC 1705, which contains no dust. When CANDELS astronomers saw that the redshift 7 galaxies in their sample have a similar color as NGC 1705, they derived that those are also dust free. The gradual reddening they observed at lower and lower redshifts reveals that the galaxies are getting dustier with time.